Last word in prayer
I believe the answer is:
'last word in prayer' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for amen that I've seen before include "Word at the end of a prayer" , "Name for end to prayer" , "Religiously 11 [THE LASTWORD]" , "Final word in prayer" , "Extremist" .)